As for the methods of forming a visible image from an image source, the methods such as an electrophotographic method, an electrostatic recording method, an electrostatic printing method and so forth in which an electrostatic latent image is used as an intermediate have heretofore widely been utilized.
In the electrophotographic method, for example, a copy image is formed in the following manner.
A uniform electrostatic charge is applied to a latent image carrying means having a light-sensitive layer made of a photoconductive material, and an electrostatic latent image corresponding to an image source is then formed on the surface of the latent image carrying means by imagewise exposure. The resulting electrostatic latent image is developed with a developer to form a toner image. The toner image is transferred to a recording material such as a paper sheet and is then heated or pressured for fixing to form a copy image. On the other hand, the latent image carrying means is electrically neutralized after completing the image transfer process and untransferred toner remaining on the latent image carrying means is cleaned off. Then, the latent image carrying means is used for the next copy image formation.
Heretofore, there have been known electrostatic image developers including, for example, a single-component type developer consisting of magnetic toners only and a two-component type developer comprising toners and carriers. Such two-component type developers have the advantage that the frictional charge of the toners can be controlled relatively easily, because the toners are charged by friction with the carriers.
In order to form excellent and stable copy images with a substantially high image density and no fog in repeated use it be required that the frictional charge of toners is constantly maintained at a proper level. To meet this requirement, it is essential to effectively prevent the carriers or the developer transporting means from contamination with toner materials.
It is preferable to fix toners by a heat- roller fixing method, because a high heat efficiency and a high-speed fixation are feasible. In the heat-roller fixing method, however, so-called offset phenomenon is liable to be caused, where a part of toners forming an image are transferred to the surface of a heat-roller when the toners are fixed, and it is further transferred to a following recording material, so that images are stained. Therefore, the toners are required to have an excellent offset resistance.
In case of forming images repeatedly, temperature of a heat-roller is liable to be lowered, because heat is transferred considerably to a recording material and, resultingly, there is liable to be caused a winding trouble where a recording material winds around surface of a heat-roller. Toners are, therefore, required to have an excellent anti-winding property.
Taking the above-mentioned requirements into consideration, there has so far been a proposal for the toner containing a releasing agent dispersed in a binder resin comprising a polyester prepared by condensation-polymerization of a monomer composition consisting of a polyvalent monomer having not less than trivalency, an aromatic dicarboxylic acid and a bisphenol type alcohol. [Refer to Japanese Patent Publication Open to Public Inspection (hereinafter called Japanese Patent O.P.I. Publication) No. 57-208559 (1985)].
However, the above-mentioned technology involves the problems. One of them is as follows: when repeating copying operation for a long time, toner particles are liable to be finely powdered because they are destroyed by external mechanical force such as agitation applied to them in a developing unit, and fine powder is fused to the surface of the carrier particles so as to make frictional electrification of both toners and carriers unstable, which in turn results in making developability unstable.
Further, another problem involved is as follows; the foregoing releasing agent dispersed in the binder resin is liable to drop off from toner particles, and the dropped releasing agent contaminates surface of a latent image carrying means, carrier particles, a developer transporting means and so forth. Such problems are liable to cause image density degradation, fogging and contamination of a machine due to toner flying. In addition, the toners without the releasing agent deteriorate the properties of fixing, anti-offset and anti-winding in a fixing process after developing and transferring.
The above-mentioned problems can be solved by a method in which an aliphatic alcohol is used as dialcohol. In this method molecular chains of polyesters become more flexible, and therefore, mechanical energy produced by agitating a developer is converted into kinetic energy of the molecular chains of the polyesters, so that the toner particles are subject to considerably reduced mechanical stress and are effectively prevented from being destroyed.
At present, however, a composite material of a binder resin and a releasing agent is not able to provide sufficient performance, while the binder resin itself is less subject to destruction in a condition of considerably intensive stress exerted on the toner particles.
In other words, there are the following problems still remaining unsolved.
(1) Polyesters for binder resins contained in toner particles become more fragile under conditions of temperature and relative humidity as low as 10.degree. C. and 20%, respectively.
Therefore, when the toner particles are Subjected to external mechanical force such as agitation in a developing unit, they are destroyed to fine powder and the fine powder is fused to the surface of the carriers, so that the frictional charge generated between toners and carriers becomes unstable, which results in making developability unstable.
As a result, the problems of the prior art remain unsolved; the releasing agent drops off and dropped contaminates the carriers and means for carrying the latent image and transporting the developer; finally, image density degradation, fog, off-set, winding of the recording material and the like are caused.
(2) When forming an image in under ordinary conditions of temperature and humidity of 20.degree. C. and RH60% in an intermittent mode where operation of a machine stops whenever image formation is finished and a toner consumption rate per agitation time of toner is relatively low, average staying time of toners in a developing unit becomes longer and agitating time of developers also becomes longer. The developers are therefore subjected to more mechanical stress, which results in destroying the toner particles to fine powders, and causing drop-off of the releasing agents, so that the same problems as mentioned above still remain unsolved.
(3) Toner blocking by dropping off of a releasing agent is sometimes noted at high temperature and humidity of 30.degree. C. and RH80%, and when using the toners for a long period of time under such conditions, they are liable to form a lump in a developing unit and to cause blank areas on copy images.